The invention relates to a lighting system which comprises an electronic operating device and at least one gas discharge lamp with filaments. In particular, the preheating operation of the gas discharge lamps is intended to be improved.
In an electronic operating device for gas discharge lamps, an AC voltage generator G which operates at a frequency which is substantially higher than the line frequency feeds energy into a load circuit. This state of affairs is illustrated in a block diagram in FIG. 1. The AC voltage generator G is connected to a load circuit comprising a lamp inductor L1, a resonance capacitor C1 and a gas discharge lamp Lp. The gas discharge lamp will be called a lamp for short in what follows. As illustrated in FIG. 1, the lamp inductor L1 and the resonance capacitor C1 mostly form a series resonant circuit which is connected to the AC voltage generator G. The lamp Lp is connected in parallel with the resonance capacitor C1. Not only is this configuration suitable for operating the lamp Lp, but it also permits the lamp to be started. Before the starting of the lamp, the load circuit constitutes a series resonant circuit of high quality. If this resonant circuit is excited with its resonant frequency, there is produced across the lamp Lp a high voltage which leads to the starting of the lamp. In order to lengthen the lifetime of the lamp, the filaments W1 and W2 of the lamp Lp must be preheated before the starting. The circuit illustrated in FIG. 1 has proved itself for the purpose of implementing preheating. The resonance capacitor C1 is not connected directly to the lamp inductor L1 and the AC voltage generator G. Rather, connection to the lamp inductor L1 is performed via the filament W1, and connection to the AC voltage generator G is performed by the filament W2. For preheating purposes, the AC voltage generator G outputs a voltage whose frequency is substantially above the resonant frequency of the series resonant circuit comprising the lamp inductor L1 and the resonance capacitor C1. The filaments W1, W2 therefore already conduct current before the starting, and are preheated. This preheating operation leads, however, to a dilemma: firstly, the preheating current must be strong enough to heat up the filaments to the required temperature in a time which is to be in the range of a second. For this purpose, the frequency of the voltage which the AC voltage generator G outputs during preheating may not be selected to be too high. Secondly, during the preheating the voltage across the lamp Lp may not be too high, since glow discharges harmful to the filaments otherwise occur. For this purpose, the frequency of the voltage which is output by the AC generator G during the preheating may not be selected to be too low. The decisive factor for this is the non-starting voltage specified by the lamp manufacturer. It may not be exceeded during preheating. For many lamps, there is no frequency for the voltage output by the AC voltage generator G during preheating for which both conditions named above are fulfilled. All possible frequencies are either too close to the resonant frequency of the series resonant circuit, comprising the lamp inductor L1 and resonance capacitor C1, and therefore produce too high a voltage across the lamp Lp, or they are too far removed from the resonant frequency and therefore produce an excessively low preheating current.
It is an object of the present invention to provide a lighting system in which the lamps can be preheated in a short time without the non-starting voltage specified for the lamps being exceeded.
According to the invention, the above-described resonance capacitor C1 is replaced by an impedance network which has the following properties: the impedance function of the impedance network has a zero point at the frequency f1. In accordance with the above statements relating to the prior art, the impedance network is connected in series to the lamp inductor L1 via the filaments W1, W2. The series circuit of the impedance network with the lamp inductor L1 has an impedance function with a zero point at the frequency f2. For preheating purposes, the AC voltage generator G now outputs a voltage whose essential spectral component is at a frequency which is near the frequency f1 for the zero point of the impedance function of the impedance network. xe2x80x9cNear the frequency f1xe2x80x9d describes in this context a frequency band from 0.8*f1 to 1.2*f1. The voltage across the lamp is thereby low (below the non-starting voltage) and at the same time it is possible to implement a sufficiently high current through the filaments W1, W2 which permits a preheating time of less than a second. For starting purposes, the AC voltage generator G outputs a voltage whose essential spectral component is at a frequency which is near the frequency f2 for the zero point of the impedance function of the series circuit comprising the lamp inductor L1 and the impedance network.
A simple configuration of the impedance network consists of the series circuit of a capacitor and an inductor. If the capacitor has the capacitance C and the inductor the inductance L, the zero point of the impedance function is at a frequency ƒ1=1/2xcfx80{square root over (LC)}.
A preheating circuit according to the invention can also be used for lighting systems with a plurality of lamps. All combinations of parallel and series connection are possible in this case. In the case of parallel connection, a plurality of lamp circuits which contain an impedance network according to the invention, a lamp inductor and a lamp are connected in parallel. In the case of series connection, only the lamps are connected in series. It then suffices to connect the impedance network according to the invention with in each case one filament terminal of the first and last lamp of the series circuit of lamps.